Apolipoprotein A-I regulates lipid hydrolysis by hepatic lipase.

Association of hepatic lipase (HL) with pure heparan sulfate proteoglycans (HSPG) has little effect on hydrolysis of high density lipoprotein (HDL) particles, but significantly inhibits (>80%) the hydrolysis of low (LDL) and very low density lipoproteins (VLDL). Lipolytic inhibition is associated with a differential ability of the lipoproteins to remove HL from the HSPG. LDL and VLDL are unable to displace HL, whereas HDL readily displaces HL from the HSPG. These data show that HSPG-bound HL is inactive. Purified apolipoprotein (apo) A-I is more efficient than HDL at liberating HL from HSPG, and HL displacement is associated with the direct binding of apoA-I to HSPG. However, displacement of HL by apoA-I does not enhance hydrolysis of VLDL particles. This appears due to the direct inhibition of HL by apoA-I. Both apoA-I and HDL are able to inhibit VLDL lipid hydrolysis by up to 60%. Inhibition of VLDL hydrolysis is associated with the binding of apoA-I to the surface of the VLDL particle and a concomitant decreased affinity for HL. These data show that apoA-I can regulate lipid hydrolysis by HL by liberating/activating the enzyme from cell surface proteoglycans and by directly modulating lipoprotein binding and hydrolysis.

Role of the kidney in regulating the metabolism of HDL in rabbits: evidence that iodination alters the catabolism of apolipoprotein A-I by the kidney.

To evaluate the factors that regulate HDL catabolism in vivo, we have measured the clearance of human apoA-I from rabbit plasma by following the isotopic decay of (125)I-apoA-I and the clearance of unlabeled apoA-I using a radioimmunometric assay (RIA). We show that the clearance of unlabeled apoA-I is 3-fold slower than that of (125)I-apoA-I. The mass clearance of iodinated apoA-I, as determined by RIA, is superimposable with the isotopic clearance of (125)I-apoA-I. The data demonstrate that iodination of tyrosine residues alters the apoA-I molecule in a manner that promotes an accelerated catabolism. The clearance from rabbit plasma of unmodified apoA-I on HDL(3) and a reconstituted HDL particle (LpA-I) were very similar and about 3-4-fold slower than that for (125)I-apoA-I on the lipoproteins. Therefore, HDL turnover in the rabbit is much slower than that estimated from tracer kinetic studies. To determine the role of the kidney in HDL metabolism, the kinetics of unmodified apoA-I and LpA-I were reevaluated in animals after a unilateral nephrectomy. Removal of one kidney was associated with a 40-50% reduction in creatinine clearance rates and a 34% decrease in the clearance rate of unlabeled apoA-I and LpA-I particles. In contrast, the clearance of (125)I-labeled molecules was much less affected by the removal of a kidney; FCR for (125)I-LpA-I was reduced by <10%. The data show that the kidneys are responsible for most (70%) of the catabolism of apoA-I and HDL in vivo, while (125)I-labeled apoA-I and HDL are rapidly catabolized by different tissues. Thus, the kidney is the major site for HDL catabolism in vivo. Modification of tyrosine residues on apoA-I may increase its plasma clearance rate by enhancing extra-renal degradation pathways.

Diacylglycerol is the preferred substrate in high density lipoproteins for human hepatic lipase.

The hydrolysis of HDL phospholipids (PL) and glycerides by hepatic lipase (HL) has been investigated in native and reconstituted HDL particles (Lp2A-I). Fasting, normolipidemic HDL exhibit total lipid hydrolytic rates of between 10 and 36 nM FA/h per microM PL. Of the total fatty acids liberated with HDL3 only 1% are from triolein (TG), while 49% are from diolein (DG) and 50% are from PL. A spherical reconstituted particle containing 2 molecules of apoA-I, 120 molecules of PL, and 20 molecules of TG exhibits a total lipid hydrolytic rate of 18 nM FA/h per microM PL and 93% of the fatty acids liberated are from PL. Inclusion of 40 molecules of TG into the Lp2A-I particle doubles the rate of fatty acid hydrolysis by HL through a stimulation of TG hydrolysis. Further addition of 10 molecules of DG to the Lp2A-I complex has no effect on the overall rates of hydrolysis, but changes the substrate specificity, wherein 61% of the fatty acids are from DG and both TG and PL hydrolytic rates are significantly reduced. Increasing the amount of DG in the Lp2A-I particle further stimulates total lipid hydrolysis by raising DG hydrolytic rates at the expense of PL and TG hydrolysis. A particle containing 10 molecules of TG and 40 molecules DG yields the fastest lipid hydrolytic rate of 143 nM FA/h per microM PL, which constitutes 96% DG hydrolysis, 3% TG hydrolysis, and 1% PC hydrolysis. These data indicate that hepatic lipase acts primarily as a surface lipid lipase with HDL particles. DG is the preferred substrate of HL in HDL and the HDL-DG content regulates the hydrolysis of both PL and TG by HL.